Arginase is an enzyme that catalyzes divalent cation-dependent hydrolysis of L-arginine to form L-ornithine and urea. Arginase is known to serve at least three important functions: (1) production of urea, (2) production of L-ornithine, and (3) regulation of arginine levels as a substrate for nitric oxide synthases (also known as NOSs, these enzymes convert L-arginine into citrulline and NO).
In most mammals, two isozymes of arginase exist: arginase I and arginase II. Arginase I is located primarily in the cytoplasm of the liver, while arginase II is found in the mitochondria of several tissues, with higher concentrations in the kidney and prostate, and lesser concentrations found in macrophages, lactating mammary glands, and the brain. The production of urea by hepatic arginase is an important mechanism to excrete nitrogen (ammonia) in the form of a highly soluble, non-toxic compound.
In tissues lacking a complete complement of the urea cycle enzymes, arginase regulates cellular concentrations of L-ornithine. L-ornithine is a precursor for the biosynthesis of polyamines (such as spermine, and spermidine, which have important roles in cell proliferation and differentiation) and proline (an important component of collagen, a component of fibrin and fibrotic tissue). Arginase also modulates NOS-mediated production of NO by regulating the levels of arginine present within tissues. In pathological disease states where extrahepatic arginases are elevated, L-arginine is more actively consumed, limiting its availability as a substrate for NOS. Arginase and NOS thus appear to be reciprocally regulated. In such disease states, it may be particularly desirable to inhibit the extrahepatic arginase.
An excess of arginase has been associated with a number of human pathological conditions, including erectile dysfunction, atherosclerosis, asthma, and pulmonary arterial hypertension and certain cancers, such as non-small-cell lung, prostate, and pancreatic cancers. Furthermore, high levels of arginase have been documented in animal models of human diseases such as myocardial ischemia-reperfusion injury, systolic (essential) hypertension, atherosclerosis, pulmonary arterial hypertension, erectile dysfunction, asthma, and multiple sclerosis.
Patients with conditions associated with an increase in arginase activity may stand to benefit from treatment with arginase inhibitors, such as NΩ-hydroxy-L-arginine (L-HO-Arg), an intermediate in the NO synthase reaction. However, L-OH-Arg is a non-selective inhibitor, and thus the exact role of arginase in pathophysiology and the potential therapeutic effects of arginase inhibitors remain unknown.
While it is desirable not to extensively inhibit hepatic arginase, there is support for the hypothesis that the urea cycle is very robust. For example, Gau and coworkers (Mol. Ther., 2009, 1:1-9) has reported that rescue of an arginase I knock-out animal by arginase I gene therapy only requires approximately 20% arginase activity in order to maintain normal ammonia levels. In other words, as long as the arginase activity in the liver does not fall below 20% normal levels, the urea cycle can function normally and hyperammonemia does not occur. In addition, the heterozygous arginase I knock-out mouse, which has only approximately 60% of normal hepatic arginase activity, has normal plasma ammonia levels as reported by Iyer and coworkers (Mol. Cell. Biol., 2002, 22:4491-4498).
There is a need in the art for inhibitors of arginase activity that may be used to treat a disease or disorder in a mammal, wherein the disease or disorder is characterized either by abnormally high arginase activity or by abnormally low nitric oxide levels in a tissue of the mammal. The present invention meets these needs.